If you get a message "need java2 for this applet" then I
recommend you go get the Java plug-in
if you are on Windows. This applet only works on java 2 because you
can't get half-decent sound without it.
This java applet demonstrates interference between two sources of
sound waves. You need two speakers (or one speaker and a wall)
and a sound card (preferably stereo).

Measure or estimate the distance between your speakers and set the Speaker
Separation slider accordingly. (Uncheck the Metric Units
checkbox if you want.) Then turn on the Sound
checkbox. The applet will play a sine wave out of both speakers,
which will create an interference pattern hopefully quite similar to
the one shown. The speakers are shown as blue dots. Plug one ear
and move your head around the room (at speaker level) to see if you
can pick up the variations
in sound intensity. They should roughly match the interference pattern,
although there may be many differences because of reflections off
walls and objects; also this applet uses a simplistic point source model
for the speakers, which will not match reality very well.

If you have a stereo sound card, then check the Stereo
checkbox. You can use the Balance slider to verify that the
stereo is working. With stereo turned on, you can adjust the relative
phase of the two speakers. By default, the speakers are 180 degrees
out of phase, so the sound intensity will be low right between them.

Note that if you have a subwoofer, then low frequencies will be coming
out of the subwoofer rather than the main speakers, so the interference
pattern won't match your experience.

You can measure distances on the view by clicking the mouse and
dragging.

You can also view the interference pattern between one speaker and a wall
by setting the Speaker Separation to twice the distance between
the speaker and the wall. Uncheck the Stereo checkbox or set
the Phase Difference to zero. Imagine the wall being drawn
vertically down the middle of the screen.

It is also interesting to move the speakers right next to each other and
then set the phase difference to 180 degrees.